In semiconductor device manufacture, a processing such as etching or film formation is performed on a target object to be processed by the action of a plasma on the target object. As an example of the semiconductor device manufactured by such processing, there is known a NAND flash memory described in, e.g., Patent Document 1.
The NAND flash memory described in Patent Document 1 includes a multilayer film formed by laminating a plurality of alternating layers of a first layer having a first dielectric constant and a second layer having a second dielectric constant. The multilayer film has a stepped structure.
In order for the multilayer film to have the stepped structure in the method described in Patent Document 1, there is used a multilayer mask formed by alternately laminating two layers capable of being selectively etched.
Patent Document 1: Japanese Patent Application Publication No. 2009-170661
In the method described in Patent Document 1 described above, it takes time to form a plurality of layers constituting the multilayer mask on the multilayer film. Accordingly, the present inventors have developed a method of providing a resist mask on the multilayer film, and alternately repeating the etching of the multilayer film and the trimming of the resist mask, namely, the reduction of the width of the resist mask. The trimming of the resist mask may be carried out by exciting a plasma of oxygen gas above the resist mask, and etching the resist mask by oxygen active species such as oxygen ions and oxygen radicals.
In general, the resist mask is easily etched in a vertical direction rather than a horizontal direction in the etching of the resist mask. As a result, (amount of the resist mask etched in the vertical direction)/(amount of the resist mask etched in the horizontal direction), i.e., a trimming ratio, becomes considerably larger than 1. Assuming that the resist mask is etched at such a large trimming ratio, a thick resist mask is required for the multilayer film to have a multistage stepped shape. However, by using the thick resist mask, the material cost may increase, and the exposure accuracy of the resist material may decrease. Alternatively, a method for improving the trimming ratio by performing a deposition process for forming a protective film on the resist mask before the etching of the resist mask may be considered. However, in this method, the throughput is reduced because of the deposition process.
Thus, in the technical field, it has become necessary to make the trimming ratio close to 1 at a high throughput in the etching of the resist mask for forming a stepped shape in the multilayer film.